The effect of protein supplementation of a wheat straw diet on the metabolism of lysine, leucine, methionine and urea,  and on whole-body protein turnover rate was investigated in lambs. The metabolism of lysine and leucine is reported elsewhere (Cronje et aI., 1992); in this paper methionine metabolism is discussed, and estimates of whole-body protein turnover derived from all three amino acids are compared. Methionine flux rate was determined from a 12-h intravenous infusion of L-[3SS] -methionine. Methionine flux, oxidation and incorporation into protein were increased (P < 0.01) by protein supplementation, but the proportion of flux oxidized or incorporated into protein was not significantly increased (P > 0.05). Methionine flux was used with a high efficiency for protein synthesis (92%). The results indicate that the supply of methionine was limiting for protein synthesis – even with the high-protein diet. Estimates of wholebody protein turnover derived from the various tracers did not differ significantly for the low-protein diet (P> 0.05) but all estimates for the high-protein diet were significantly different (P < 0.06). The same trend was apparent in the case of whole-body protein synthesis, but leucine and methionine provided similar estimates for the high-protein diet. It was suggested that, although much stands to be gained from studies of the metabolism of specific amino acids under such dietary conditions, the use of such data to calculate whole-body protein metabolism should be approached with caution. The protein metabolic status of the animals in this experiment was characterized as one of primary protein deficiency, exacerbated by amino acid specific deficiencies (methionine and lysine). It was suggested that substantial potential exists for improvement of the efficiency of utilization of existing protein supplements for roughage diets by supplementation with specific amino acids protected from degradation in the rumen, particularly in young growing lambs.